The following account of the prior art relates to one of the areas of application of the present invention, hearing aids.
In hearing aids, acoustic feedback from the receiver to the microphone(s) may lead to howl. In principle, howls occur at a particular frequency if two conditions are satisfied:                a) The loop gain exceeds 0 dB.        b) The external signal and feedback signal are in-phase when picked up by the microphone.        
WO 2007/006658 A1 describes a system and method for synthesizing an audio input signal of a hearing device. The system comprises a filter unit for removing a selected frequency band, a synthesizer unit for synthesizing the selected frequency band based on the filtered signal thereby generating a synthesized signal, a combiner unit for combining the filtered signal and the synthesized signal to generate a combined signal.
US 2007/0269068 A1 deals with feedback whistle suppression. A frequency range which is susceptible to feedback is determined. From an input signal which has a spectral component in the frequency range susceptible to feedback, a predeterminable component is substituted with a synthetic signal.
WO 2008/151970 A1 describes a hearing aid system comprising an online feedback manager unit for—with a predefined update frequency—identifying current feedback gain in each frequency band of the feedback path, and for subsequently adapting the maximum forward gain values in each of the frequency bands in dependence thereof in accordance with a predefined scheme.
WO 2007/112777, and WO 94/09604 describe various estimators of loop gain as a function of frequency.